1. Field of the Invention
The present invention relates to a technique for uniformly irradiating a laser beam to a large area. In addition, the invention relates to its application.
2. Description of the Related Art
In recent years, a wide research has been made on a technique for carrying out a laser annealing to an amorphous semiconductor film or a crystalline semiconductor film (semiconductor film having crystallinity, such as polycrystal or microcrystal, not single crystal), that is, a non-single crystal semiconductor film formed on an insulating substrate of glass or the like to crystallize it or to improve its crystallinity. A silicon film is often used as the above semiconductor film.
As compared with a quartz substrate which has been hitherto often used, a glass substrate has such merits that it is inexpensive and is rich in workability, and a large substrate can be easily formed. This is the reason why the foregoing research has been carried out. Besides, the reason why a laser is preferably used for crystallization is that the melting point of the glass substrate is low. The laser can give high energy to only a non-single crystal film without greatly changing the temperature of the substrate.
Since a crystalline silicon film formed by the laser annealing has a high mobility, a thin film transistor (TFT) is formed by using this crystalline silicon film. For example, it is actively used for a monolithic liquid crystal electro-optical device in which TFTs for pixel driving and for driver circuits are formed on one glass substrate. Since the crystalline silicon film is made of a number of crystal grains, it is called a polycrystal silicon film or polycrystal semiconductor film.
A method in which a pulse laser beam of an excimer laser or the like having high output is processed by an optical system so that a rectangular spot of several cm or a linear beam of several mm in width x not less than 10 cm in length is formed on a surface to be irradiated, and the laser beam is scanned (irradiation position of the laser beam is relatively moved with respect to the irradiated surface) to carry out the laser annealing, is superior in mass productivity and is excellent in technology. Thus, this method is used by choice.
Particularly, when a linear laser beam is used, unlike the case of using a spot-like laser beam which requires back-and-forth and right-and-left scanning, laser irradiation to the whole surface to be irradiated can be made by scanning in only the direction perpendicular to the line direction of the linear laser. Thus, high mass productivity can be obtained. The reason why scanning is made in the direction perpendicular to the line direction is that it is the most effective scanning direction. Because of this high mass productivity, at present, in the laser annealing, it has become the mainstream to use the linear laser beam obtained by processing the excimer laser beam through a suitable optical system.
Recently, a continuous-wave laser, such as an Ar laser, having a higher output has been developed. There is also a report that an Ar laser was used for annealing of a semiconductor film and an excellent result was obtained. In this case, since the output of the Ar laser is not sufficient, an irradiation surface has a spot shape.
As a laser widely used for crystallization, an excimer laser is known as a gas laser, and a ND:YAG laser, Nd:YVO4 laser, or argon laser is known as a solid laser.
Since the continuous-wave argon laser has a wavelength of about 500 nm, the absorption coefficient of the argon laser to a silicon film is about 105/cm. On the other hand, since the excimer laser is ultraviolet light of 400 nm or less, the absorption coefficient is about 106/cm which is higher than the argon laser by one digit. Thus, in the argon laser, the intensity is decreased to 1/e (e is a natural logarithm) at the point when the light travels 100 nm through the silicon film, while in the excimer laser, the intensity is decreased to 1/e at the point when the light travels 10 nm through the silicon film.
In general, in the field of a TFT, it is considered to be suitable that the thickness of a polycrystal silicon film is about 50 nm. If the silicon film has a thickness more than 50 nm, there is a tendency that off characteristics become deteriorated, and if less than 50 nm, the reliability is influenced. In the case where the argon laser light is irradiated to the silicon film having a thickness of 50 nm, more than half of the laser light transmits the silicon film and is absorbed by the glass substrate, so that the glass substrate is heated more than needed. Actually, when a silicon oxide film having a thickness of 200 nm and a silicon film having a thickness of 50 nm were sequentially formed on a Corning 1737 substrate and crystallization was attempted, the glass was deformed before the silicon film was sufficiently crystallized.
On the other hand, in the case of irradiation of the excimer laser, almost all energy is absorbed by the silicon film having a thickness of 50 nm, so that almost all laser light can be used for crystallization of the silicon film. Like this, the merit of using the excimer laser for crystallization of the silicon film is that the absorption coefficient of the excimer laser to the silicon film is high.
FIG. 24A is a top view of a silicon film, which is irradiated with a conventional pulse oscillation excimer laser while being scanned, seen from the above. FIG. 24B is a sectional view of the silicon film cut along a section (surface perpendicular to the silicon film containing a line E-F) parallel to the scanning direction of the pulse oscillation excimer laser. FIG. 24C is a sectional view of the silicon film cut along a surface (surface perpendicular to the silicon film containing a line G-H) perpendicular to the section and perpendicular to the silicon film.
As is understood from FIG. 24B, irradiation traces of the pulse laser produce undulations of the same order as the thickness of the silicon film. On the other hand, although the undulations shown in FIG. 24C are vary small as compared with the undulations of FIG. 24B, periodic undulations appear. These are due to the interference of linear beams shaped by a beam homogenizer as described later.
An optical system serving to homogenize an energy distribution (light intensity) in a linear beam is called a beam homogenizer. FIGS. 25A and 25B show an example of the beam homogenizer.
On an optical path, cylindrical lens groups (also called a multi-cylindrical lens or cylindrical lens array) 12 and 13, a cylindrical lens 14, a slit 15, a cylindrical lens 16, and a mirror 17 are sequentially disposed from an outgoing side of a laser apparatus 11 as an optical source of an excimer laser. A cylindrical lens 18 is disposed on an optical path in the reflecting direction of the mirror 17.
The cylindrical lens 12 divides a beam into plural beams in a predetermined one direction (direction perpendicular to the paper surface of the side view), and the beams divided in this direction are synthesized in the cylindrical lens 16. On the other hand, the cylindrical lens group 13 also divides a beam into plural beams in a predetermined one direction (direction parallel to the paper surface of the side view), and the beams separated in the dividing direction of the cylindrical lens group 13 are synthesized in the cylindrical lens 14.
Thus, the laser beam emitted from the oscillator is divided two-dimensionally by the cylindrical lens groups 12 and 13, and is inputted on the cylindrical lens 14. Some of the plural beams are synthesized in the predetermined direction (direction perpendicular to the paper surface of the side view) so that plural beams divided in the one direction (direction parallel to the paper surface) are formed and pass through the slit 15. The beams are condensed by the cylindrical lens 16 so that they become again one beam. The condensed beam is reflected by the mirror 17, is condensed by the cylindrical lens 18, and is irradiated as a linear beam 19 (direction perpendicular to the paper surface of the side view is a longitudinal direction).
In the homogenizer of FIGS. 25A and 25B, the dividing directions of the beam in the cylindrical lens groups 12 and 13 cross at right angles, and the condensing directions of the beam in the cylindrical lenses 14 and 16 cross at right angles. The intensity distribution of the linear laser beam 19 in the longitudinal direction is unified by the combination of the cylindrical lens group 12 and the cylindrical lens 16. The intensity distribution of the linear laser beam 19 in the width direction is unified by the combination of the cylindrical lens group 13 and the cylindrical lens 14. That is, the beam is divided two-dimensionally and is again synthesized, so that the energy of the linear beam is unified.
Thus, it appears that as the number of beams divided by the cylindrical lens groups 12 and 13 becomes large, the distribution of energy becomes uniform. However, irrespective of the fineness of division, stripe patterns of irradiation traces of the linear laser beam were formed on the silicon film. As shown in FIG. 24A, the countless stripe patterns appear to be orthogonal to the longitudinal direction of the linear laser beam (scanning direction of the linear beam, direction of GH), and peaks appear periodically on the silicon film as shown in FIG. 24C. It is expected that the cause of the stripe patterns is either one of the beam before it is incident on the beam homogenizer and the optical system of the beam homogenizer.
The present inventor carried out a simple experiment to find the cause of the stripe patterns. An examination was made on the change of the stripe patterns caused by rotating a laser beam before the rectangular laser beam was incident on the beam homogenizer. The result was that the stripe patterns did not change at all. It has been found that the cause of the stripe patterns is not the beam before it is incident on the beam homogenizer but the beam homogenizer. In the beam homogenizer, since a beam with a single wavelength and equal phases (since a laser obtains the intensity by equalizing the phases, the phases of light are naturally equalized) is divided and is again superimposed to unify the energy, it is permissible to explain that the stripe patterns are optical interference fringes when light is superimposed.
An object of the present invention is to solve the foregoing problem of interference of beams having equal phases, such as laser light, and to unify the energy distribution of the linear laser light in the longitudinal direction.
A beam homogenizer of the invention comprises a first dividing optical lens for dividing one beam into (2n+1) beams in a first direction, a second dividing optical lens for dividing one beam into N(nxe2x88x921) beams in a second direction perpendicular to the first direction, a first synthesizing lens for condensing light in the second direction and for synthesizing the plurality of beams divided in the second direction, and a second synthesizing lens for condensing light in the first direction and for synthesizing the plurality of beams divided in the first direction,
wherein the second synthesizing lens includes (nxe2x80x2xe2x88x921) cylindrical lenses,
wherein images obtained by orthogonal projection of respective principal points of the (nxe2x80x2xe2x88x921) cylindrical lenses onto a plane orthogonal to the second direction become (nxe2x80x2xe2x88x921) points arranged with an interval of d/(nxe2x80x2xe2x88x921) on a same line,
wherein the character d designates an interval of peaks of interference fringes formed on an irradiation surface by the beam passing through one cylindrical lens of the second synthesizing lens, and
wherein the character N designates a natural number, the character n designates an integer not less than 3, and the character nxe2x80x2 designates an integer satisfying 3xe2x89xa6nxe2x80x2xe2x89xa6n.
When the phases of the plurality of linear laser beams in the longitudinal direction are shifted by a predetermined size and are synthesized, the intensity of the interference fringes on the irradiated surface of the linear beams can be made uniform, as shown in FIGS. 28A-28C and 29A-29D explained later.
When a mirror is inserted in the beam homogenizer of the invention, since the direction of an optical path is changed, the dividing direction of the beam by the cylindrical lens or the condensing direction is also changed. However, in the invention, the change of direction by the mirror is neglected, and the case including no mirror is assumed.
According to the invention, in the second synthesizing lens, the phases of the plurality of beams are shifted, and the plurality of beams are condensed so that they are irradiated to the same region. Thus, in the second synthesizing lens, the principal points of the respective cylindrical lenses are shifted by d/(nxe2x80x2xe2x88x921) in the direction perpendicular to the optical axis.
The first dividing lens of the invention includes a cylindrical lens group in which (2n+1) cylindrical lenses with optical axes parallel to each other are coupled into a column shape (array shape). Moreover, here, although a beam is divided into an odd number (2n+1) beams in the first dividing cylindrical lens group, it may be divided into to each other may be coupled into a column shape.
The second dividing lens of the invention may be constructed by a cylindrical lens group in which N(nxe2x88x921) cylindrical lenses with optical axes parallel to each other are coupled in a column shape. The first synthesizing lens may use a cylindrical lens.
The homogenizer of the invention shows remarkable effects in the case where coherent beams are linearly shaped, and the light intensity of the linear beam in the longitudinal direction can be smoothed. As a light source of the coherent light, a laser apparatus such as a gas laser or solid laser is used. A continuous-wave argon laser apparatus or pulse oscillation type excimer laser apparatus may be used.
As the gas laser, an excimer laser may be named. Although the excimer laser is widely recognized as a pulse oscillation type laser, a continuous-wave excimer laser oscillation apparatus has been developed recently. In order to make continuous light emission, a microwave is used to accelerate excitation of an oscillation gas.
By irradiating the microwave of the order of giga hertz to the oscillation gas to promote a rate determining reaction of oscillation, it has become possible to make continuous light emission of the excimer laser. The excimer laser having a high absorption coefficient to a silicon film becomes more and more important for crystallization of a semiconductor film when a continuous-wave one is put into practical use. When the continuous-wave excimer laser is used, irradiation traces of a pulse laser can be eliminated, so that the effect of laser irradiation processing can be greatly made uniform.
As the excimer laser, for example, a KrF laser (wavelength 248 nm), XeCl excimer laser (wavelength 308 nm), XeF laser (wavelength 351 nm, 353 nm), ArF excimer laser (wavelength 193 nm), XeF laser (wavelength 483 nm), or the like may be used.
As the solid laser, a pulse oscillation type Nd:YAG laser or Nd:YVO4 laser may be used. Especially when a pulse oscillation laser apparatus of laser diode excitation system is used, a high output and high pulse oscillation frequency can be obtained. Although the basic frequency of the Nd: YAG laser or Nd: YVO4 laser is 1064 nm, not only the basic frequency but also either one of the second harmonic (532 nm), third harmonic (354.7 nm), and fourth harmonic (266 nm) may be used.